244 



LECTURE XT. 



" In the SyllidtB the branchial organs are penetrated by the peri- 

 toneal fluid; but it can be detected in motion only in the bases of the 

 feet, and these parts only are furnished with vibratile cilia, which are 

 large and active. The long filiform and, in some species, moniliform 

 appendages, which are described commonly as the branchiae of these 

 worms, have no central hollow ; they are filled with large-celled tissues, 

 through which the fluid parts of the contents of the visceral cavity 

 slowly penetrate. In the Syllidan family, which excels all others in 

 grace and beauty, the proper blood-system is almost indetectible, in 

 consequence of the colourlessness of the contents. It may be stated 

 with confidence, that blood-vessels do not enter into the structure of 

 the branchial processes. The respiration therefore devolves exclu- 

 sively on the chyle-aqueous fluid. 



" Amongst the family Ariciadce several other varieties in the con- 

 figuration of the breathing organs occur. In the genera Leucodore, 

 Nerine, and Aricia, the branchial appendages affect a dorsal situation. 

 In every species they are traversed from base to apex by a single 

 blood-vessel returning upon itself. This vessel, however, is sup- 

 ported by a lobule of spongy tissue, into the cells of which the fluid 

 of the visceral chamber penetrates. The ofiice of respiration in this 

 family is therefore discharged in part by the blood and in part by the 

 chyle-aqueous fluid. In very species of this family the branchice are 

 supplied by vibratile cilia having a distinct disposition in each. Leu- 

 codore ciliatus, on the dorsal aspect and over the posterior two- 

 thirds of the body, is covered on each side with a row of flattened 

 conical branchial processes, blood-red in colour and richly ciliated. 

 They are largest anteriorly, and smallest near the tail. The cilia are 

 disposed in a spiral line, from the attached to the extreme end. 

 Viewed with a high magnifying power, and transparently, a camerated 

 axis, composed of exquisitely fine hyaline cartilage, may be dis- 

 covered, fulfilling on the branchiae of this elegant little boring Anne- 

 lid the ofiice of mechanical suj^port, as a similar structure was for- 

 merly shown to do in those of the Sabellce. 



" In the genus Nerine the respiratory organs occur under forms of 

 the highest beauty. They constitute flat, membranous, penknife- 

 shaped appendages, curving gracefully over the back with the curve 

 of the " ring " of the body by which they are supported, and crossing 

 over the dorsal median line, and alternating with the corresponding 

 process of the other side. The plane of each process is vertical in 

 relation to the longitudinal axis of the body ; they lie, therefore, one 

 over the other in an imbricate manner. They are less flat and close 

 in N. vulgaris than in N. coiiiocephala. They are largest in size 

 towards the middle of the body, smallest anteriorly and posteriorly. 



